In the case of a common rail injection system, the fuel is injected into the combustion chamber of an internal combustion engine at a pressure of up to 2000 bar. The high fuel pressure requires precise control of the injection time and of the injection quantity. Furthermore, it is necessary, for internal combustion engines which are operated with diesel fuel, to carry out an exact pre-injection with a small quantity of fuel in order to minimize the noise of the internal combustion engine and also the emission of pollutants. For the abovementioned reasons, it is necessary to coordinate the injection valve very precisely, so that an optimum shaping of the injection profile is achieved.
The article “A Common Rail Injection System For High Speed Direct Injection Diesel Engines”, SAE paper 980803, by N. Guerrassi et al. discloses a fuel injection valve for a common rail injection system which has a control chamber which is supplied with fuel by a fuel line via a inlet throttle. The control chamber is connected via a outlet throttle to a outlet line which can be connected to a fuel reservoir via an electromagnetic valve. Furthermore, a bypass throttle is provided which creates a connection between the fuel line and the outlet line. The control chamber is bounded by a nozzle needle which is arranged in an axially movable manner in a nozzle body. The nozzle needle is guided through a nozzle chamber which is connected to the fuel line. Furthermore, the nozzle needle has pressure surfaces which are acted upon by the fuel pressure prevailing in the nozzle chamber and apply force to the nozzle needle in the direction of the control chamber. A nozzle spring which prestresses the nozzle needle in the direction of its sealing seat is provided in the control chamber. The pressure in the pressure chamber is controlled as a function of the opening position of the electromagnetic valve. If the valve is opened, fuel flows out of the pressure chamber via the outlet throttle and at the same time less fuel flows in via the inlet throttle, so that the pressure in the control chamber drops. As a consequence of this, the nozzle needle is moved in the direction of the nozzle chamber, the nozzle needle lifting with its point off a sealing seat and releasing a connection between the fuel line and injection holes.
If the electromagnetic valve is now closed, then fuel flows into the control chamber via the inlet throttle, via the bypass throttle and the outlet throttle. In this manner, the pressure in the control chamber is rapidly increased, so that the nozzle needle is pressed relatively rapidly onto its sealing seat in the nozzle body and the injection is therefore rapidly ended.
The injection valve described has the disadvantage of the nozzle spring being situated in the control chamber and hence a relatively large control chamber being necessary, which constitutes a large harmful volume. Furthermore, the installation of the nozzle spring in the control chamber gives rise to the risk of, during installation, particles of dirt entering into the control chamber and collecting in the outlet throttle and impairing the functioning capability of the injection valve. Cavitation bubbles arising in the inlet throttle may damage the nozzle spring.